In general, when a compound is used as an effective active ingredient of a pharmaceutical product, the compound is required to be chemically and physically stabile so as to maintain a quality stably, and/or to facilitate storage management. Therefore, the resulting compound is preferably in the form of a stable crystal, and usually, there are many instances in which ultrastable crystals are selected as the drug substances for pharmaceutical products.
However, Patent Document 1, Non-Patent Document 1, and Non-Patent Document 2 describe that 1-(2′-cyano-2′-deoxy-β-D-arabinofuranosyl)cytosine monohydrochloride, which is a pyrimidine nucleoside derivative, represented by the following formula (1):

shows an in vitro suppressive effect for proliferation of human or mouse tumor cells, and also shows an excellent anti-tumor activity in vivo.
Methods for producing the compound have been reported, for example, a method of dissolving 1-(2′-cyano-2′-deoxy-β-D-arabinofuranosyl)-N4-acetylcytosine represented by the following formula (2):

in a methanol solution of hydrochloric acid, allowing the compound to react while stirring at room temperature, and after completion of the reaction, crystallizing the product from ethanol and ether (Non-Patent Documents 1 and 2); and a method of heating the compound represented by the above formula (2) to reflux in acetic acid to thereby subject the compound to de-N-acetylation, subsequently obtaining 1-(2′-cyano-2′-deoxy-β-D-arabinofuranosyl)cytosine represented by the following formula (3):

by silica gel column chromatography, dissolving the compound in a methanol solution of hydrochloric acid, allowing the compound to react while stirring at room temperature, and after completion of the reaction, crystallizing the product from ethanol and ether (Patent Document 1).
The crystal obtainable by these methods was thought to be a ½ ethanolate at the time when the document was reported, and the melting point of the crystal was considered to be 175° C. to 176° C. (Patent Document 1 and Non-Patent Documents 1 and 2). However, other than this melting point, no specific reports with regard to the crystal polymorphism or stability have been reported.